This invention relates to flash analog-to-digital (A/D) converters and, in particular, to superconductive flash A/D converters employing Josephson junctions.
Josephson junctions have a current-voltage (I-V) characteristic of the type shown in FIG. 1 and, for ease of illustration, in the appended drawings, Josephson junctions are shown by an "X". Initially, prior to being powered, each Josephson junction is in a "superconductive" state (S-state) and functions as a short circuit, i.e., it's resistance [and/or impedance] is zero. A Josephson junction remains in the S-state until the current through the Josephson junction exceeds the critical current (Ic) of the device. When the critical current (Ic) of the Josephson junction is exceeded, the device is then switched to what is termed the "normal" state (N-state). In going from the "S" state to the "N" state, the characteristic of the device changes abruptly as shown in FIG. 1. In the "S" state, a Josephson junction exhibits zero impedance and zero voltage drop for current through the device below the critical current (Ic) of the device. In the "normal" state which may also be termed the "voltage" or "resistive" state, the Josephson junction exhibits a very high impedance for voltages of less than, for example, 2.5 millivolts and somewhat lower but still significant impedance for voltages in excess of, for example, 2.5 millivolts applied or developed across the Josephson junction.
A superconducting quantum interference device (SQUID) is a circuit which includes one or more Josephson junctions and one or more inductive loads. A single junction SQUID includes the combination of a single Josephson junction connected across an inductance, as shown in FIG. 2A. A current may be injected into one end of the inductance and Josephson junction combination, and the other end of the combination may be returned to ground or some point of reference potential.
A very significant property of the single junction SQUID, from the stand point of A/D conversion, is to be found in the relationship between the magnetic flux in the SQUID and the value of the injected current. As shown in FIG. 2B, the SQUID exhibits a periodic transfer function. The output periodicity of the transfer function, which may be represented as a somewhat "skewed" sine wave, is a function of the amplitude of the current injected in the SQUID. Furthermore, the periodicity of the transfer function doubles when the current through the SQUID doubles. This property of the single junction SQUID is used in the design and operation of A/D converters embodying the invention.
In known superconductive A/D designs, several high speed signals are necessary to operate the circuit. Many of these signals also require amplitude levels precise to one part in 2.sup.n for an n-bit converter. At frequencies above 10.sup.9 Hz, this becomes difficult and expensive to control due to the high cost of the microwave equipment necessary to allow independent adjustment of all these signal levels with precision. These problems are avoided in circuits embodying the invention.